Mitochondrial Depolarization in Glutamate-Stimulated Neurons: An Early Signal Specific to Excitotoxin Exposure

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Abstract
A brief exposure to high concentrations of glutamate kills cultured forebrain neurons by an excitotoxic process that is dependent on Ca2+influx through the NMDA receptor. In this study, we have measured striking changes in mitochondrial function during and immediately after intense glutamate receptor activation. Using indo-1 microfluorometry and a specific inhibitor of the mitochondrial Na+/Ca2+exchanger, CGP-37157, we have demonstrated that mitochondria accumulate large quantities of Ca2+during a toxic glutamate stimulus and further that Ca2+efflux from mitochondria contributes to the prolonged [Ca2+]ielevation after glutamate removal. We then used JC-1 (5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolocarbocyanine iodide), a ratiometric indicator of mitochondrial membrane potential (Δψ), to show that Ca2+accumulation within the organelle dissipates Δψ. The abrupt loss of Δψ after glutamate stimulation did not occur in the presence of MK801 or in the absence of extracellular Ca2+. The mitochondrial depolarization was also cyclosporin A-sensitive, indicating a probable role for the permeability transition pore. Hence mitochondrial Ca2+accumulation and the subsequent permeability transition may be a critical early event specific to the NMDA receptor-mediated excitotoxic cascade.